The measurement of the third dimension of a surface is becoming increasingly important in many areas of medical technology. By way of example, in minimally invasive surgery the absence of the size and distance estimation via the direct view has to be replaced by measuring methods. Furthermore, surface data obtained for example within the abdomen during an operation can be matched with data recorded by other diagnostic methods, such as, for example, magnetic resonance, computed tomography or ultrasound methods, in order to better identify or localize organs or diseased tissue. Changes as a result of a “new” position of the patient during an operation or as a result of periodic position changes caused by respiration, for example, are likewise intended to be taken into account. Conventionally there are numerous 3D measuring methods, such as, for example, phase-coded active triangulation or laser scanning, which are suitable in principle for the application described. However, these methods are tailored to the measurement of non-transparent surfaces such as often occur in industrial metrology. However, organic tissues have a significantly more complex interaction with light, are partly wave-dependently transparent and have a light scattering capability in the volume which significantly changes the structure of a projected pattern and makes it difficult to recognize in a camera image for 3D data reconstruction in active triangulation methods. As a result, gaps arise in the 3D surface or the measurement uncertainty can increase greatly.
In the area of dental medicine, monochromatic light is conventionally used for phase triangulation and a white colorant is sprayed on in order to prevent light from penetrating into the tooth enamel. For the patient this is an unpleasant additional process step that adversely affects acceptance of the method.
A method better suited to 3D applications appertaining to medical technology is disclosed in WO 01/48438. This disclosure proposes providing a particularly compact and therefore disturbance-proof color pattern for a coding by means of the variation of a two-dimensional color pattern consisting of colored pattern elements. The aim is to determine a displacement position for a pattern element in the image recording of the two-dimensional color pattern projected onto an object. The three-dimensional data of an object point can be calculated by means of subsequent triangulation with a known position of the projector in a camera.
Originally, color coded triangulation (CCT) was likewise developed for applications appertaining to medical technology and affords significant advantages in the measurement of semitransparent diffusely scattering media. Applications may be a three-dimensional measurement of the human face for biometric use in the cosmetics industry, three-dimensional scanning of ear impressions, in order to produce hearing aids which are optimally adapted by means of these data, or direct scanning of the surface of the auditory canal using a specially developed CCT scanner. The advantage of this measuring method is that it affords many advantages of 3D measurement by means of active triangulation and moreover is very fast and comparatively robust. Fast means that it is able to measure in real time, since only one image recording is required for the reconstruction of the three-dimensional data sets. Robust means that, as a result of the use of the color coding of the projected pattern, it enables a comparatively good data reconstruction even in the case of biological surfaces, since it searches for color transitions or color edges during decoding and dispenses with the purely intensity-based data reconstruction.
Color triangulation CCT hitherto has involved choosing color fringe patterns having identical fringe widths for all colors. This is an expedient approach in the case of objects having no or a very small penetration depth into the object medium and the value of the modulation transfer function is virtually identical for all colors (light wavelengths). In the case of biological objects, the modulation transfer function (MTF) falls with the degree of volume scattering particularly at high spatial frequencies. The volume scattering tends to increase with the wavelength. In actual fact the effect is also dependent on the layer structure of, for example, human skin. Corresponding scatterings are not taken into account for the design of conventional color patterns.
In the dental area, the surface of teeth is scanned for adapting accurately fitting crowns, etc. In this case, too, it has been found that volume scattering makes it difficult to register valid measurement points.